A year later, after a wave of harassment claims against powerful men in entertainment, politics, the arts and the news media, the discussion has come full circle with President Trump criticizing the latest politician exposed for sexual misconduct even as he continues to deny any of the accusations against him.

In this case, Mr. Trump focused his Twitter-fueled mockery on a Democratic senator while largely avoiding a similar condemnation of a Republican Senate candidate facing far more allegations. The turn in the political dialogue threatened to transform a moment of cleansing debate about sexual harassment into another weapon in the war between the political parties, led by the president himself.

Indeed, Republicans on Friday were more than happy to talk about Senator Al Franken, Democrat of Minnesota, who apologized this week after a radio newscaster said he forcibly kissed her and posed for a photograph a decade ago appearing to fondle her breasts while she was sleeping. Democrats, for their part, sought to keep the focus on Roy S. Moore, the Republican candidate in Alabama who has been accused of unwanted sexual conduct by multiple women going back even further, including one who was 14 at the time.

But the notion that Mr. Trump himself would weigh in given his own history of crude talk about women and the multiple allegations against him surprised many in Washington who thought he could not surprise them anymore. A typical politician with Mr. Trump’s history would stay far away from discussing someone else’s behavior lest it dredge his own back into the spotlight. But as Mr. Trump has shown repeatedly during his 10-month presidency, he is rarely deterred by conventional political wisdom even as he leaves it to his staff to fend off the cries of hypocrisy.

“Like everything else Trump touches, he hijacks it with his chronic dishonesty and childishness,” said Mark Salter, a longtime adviser to Senator John McCain, Republican of Arizona. “The intense, angry and largely ignorant tribalism afflicting our politics predates Trump’s arrival on the scene. But he has infused it with a psychopath’s inability to accept that social norms apply to him.”

White House aides labored on Friday to distinguish Mr. Trump’s case from those of others, arguing that the president’s conduct was not at issue because he won the election last year after voters had a chance to evaluate both the claims against him and his denials.

“This was covered pretty extensively during the campaign,” said Sarah Huckabee Sanders, the White House press secretary. “We addressed that then. The American people, I think, spoke very loud and clear when they elected this president.”

She added that Mr. Trump still maintained that the more than a dozen women who have said that he kissed or groped them against their will were all lying. And she acknowledged no double standard in the president chastising others for sexual misconduct.

“Senator Franken has admitted wrongdoing and the president hasn’t,” she said. “I think that’s a very clear distinction.”

But Democrats saw the distinction differently. Hillary Clinton said Mr. Franken’s apology and call for an ethics committee investigation “is the kind of accountability I’m talking about — I don’t hear that from Roy Moore or Donald Trump.” Speaking with Rita Cosby on WABC Radio, Mrs. Clinton added, “Look at the contrast between Al Franken, accepting responsibility, apologizing, and Roy Moore and Donald Trump, who have done neither.”

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Donald Trump’s Lewd Comments About Women

In a 2005 recording obtained by The Washington Post before the presidential election, Donald J. Trump talks about women in vulgar terms to Billy Bush, then the host of “Access Hollywood.”

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In a 2005 recording obtained by The Washington Post before the presidential election, Donald J. Trump talks about women in vulgar terms to Billy Bush, then the host of “Access Hollywood.”CreditCredit...Mark Makela for The New York Times

For her own part, the sexual harassment conversation has been uncomfortable for Mrs. Clinton as well. Conservatives defending Mr. Moore point to various allegations made against Bill Clinton when he was president, including sexual assault, and even some liberals said they should rethink their defense of the 42nd president.

Senator Kirsten Gillibrand, a Democrat from New York who succeeded Mrs. Clinton in the Senate, told The New York Times on Thursday that in retrospect, Mr. Clinton should have resigned during the Monica S. Lewinsky scandal, which led to the president’s impeachment by the House and acquittal by the Senate.

That drew a sharp retort from Mrs. Clinton’s longtime adviser, Philippe Reines. “Senate voted to keep POTUS WJC,” he wrote on Twitter. “But not enough for you @SenGillibrand? Over 20 yrs you took the Clintons’ endorsements, money, and seat. Hypocrite.”

Mr. Trump jumped into the sexual harassment debate about 10 p.m. on Thursday. “The Al Frankenstien picture is really bad, speaks a thousand words,” he wrote on Twitter, misspelling Frankenstein. “Where do his hands go in pictures 2, 3, 4, 5 & 6 while she sleeps?”

Mr. Trump has said nothing on Twitter about Mr. Moore, who has been abandoned by many national Republicans. When the allegations arose, Mr. Trump was in Asia and he had Ms. Sanders issue a statement saying he hoped that “if these allegations are true, Judge Moore will do the right thing and step aside.” Asked by reporters on Air Force One about the case, Mr. Trump said he did not know much about it and would comment once he returned to Washington.

Since returning, however, he has said nothing, leaving it instead to Ms. Sanders. “The president certainly finds the allegations extremely troubling,” she said on Friday, adding that it would be up to “the people in the state of Alabama” to decide “whether or not they support and vote for Roy Moore.”

Unlike her father, Ivanka Trump has not hesitated to condemn Mr. Moore. “There’s a special place in hell for people who prey on children,” she said in an interview with The Associated Press this week. “I’ve yet to see a valid explanation and I have no reason to doubt the victims’ accounts.”

Kellyanne Conway, the president’s counselor, explained the president’s decision to speak out about Mr. Franken and not Mr. Moore as a function of the news cycle. “Al Franken was a brand new news story yesterday and the president weighed in as he does on the news of the day, often enough,” she said on Fox News. “The Roy Moore story is eight days old and the president put out a statement during his Asia trip on that.”

Other Republicans shook their heads. “It’s surprising to me that he would draw attention to this given the conversation about him, even if it’s all untrue,” said Sara Fagen, a White House political director under President George W. Bush. “This is one you send your vice president on.”

The president’s comment on Mr. Franken inevitably led television networks to replay his now infamous “Access Hollywood” tape, in which he was recorded saying that when he saw beautiful women, “I just start kissing them” and would try to “grab ‘em” by their private parts. “And when you’re a star, they let you do it,” he said. “You can do anything.”

He apologized, saying those remarks did not reflect his real self, but later dismissed them as “locker room talk.” A succession of women, however, accused him of grabbing their breasts, trying to put his hand up a skirt and other sexual misconduct. Mr. Trump called such claims “lies” but never followed up on his threat to sue them.

And so the president’s behavior has become wrapped up in the larger national conversation on women, power and sex that arguably started, in its current incarnation, with his campaign a year ago, a conversation that Ms. Sanders said “should be taken very seriously.” But the nation’s leader is a compromised figure when it comes to that discussion.

“A president should be a step above in leading for the entire country,” Ms. Fagen said. “When somebody is behaving in such a immoral way, a president should call them out. Trump’s a unique case here. He’s got his own issues with respect to this. He denies them all but he’s got them.”